Bimodal Size Distribution of Gold Nanoparticles under Picosecond

On the basis of comparison with previous work, picosecond laser pulses are thought to be the most efficient way to cause laser-induced size reduction ...
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J. Phys. Chem. B 2005, 109, 9404-9410

Bimodal Size Distribution of Gold Nanoparticles under Picosecond Laser Pulses Susumu Inasawa,*,† Masakazu Sugiyama,‡ and Yukio Yamaguchi† Departments of Chemical System Engineering and Electronic Engineering, School of Engineering, The UniVersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: December 24, 2004; In Final Form: February 23, 2005

The evolution of size distributions of gold nanoparticles under pulsed laser irradiation (Nd:YAG, λ ) 355 nm, pulse width 30 ps) was carefully observed by transmission electron microscopy. Interestingly, the initial monomodal size distribution of gold nanoparticles turned into a bimodal one, with two peaks in the number of particles, one at 6 nm and the other at 16-24 nm. The sizes for small particles depended very little on the irradiated laser energy. This change is attributed to laser-induced size reduction of the initial gold nanoparticles followed by the formation of small particles. In our analysis, we extracted a characteristic value for the size-reduction rate per one pulse and revealed that laser-induced size reduction of gold nanoparticles occurred even below the boiling point. When laser energy is insufficient for the boiling of particles, formation of gold vapor around liquid gold drops is thought to cause the phenomenon. With enough laser energy for the boiling, the formation of gold vapor around and inside liquid gold drops is responsible for the phenomenon. We also observed particles with gold strings after one pulse irradiation with a laser energy of 43 mJ cm-2 pulse-1, which is sufficient energy for the boiling. It is considered that such particles with gold strings are formed by the projection of gaseous gold from liquid gold drops with some volume of liquid gold around the bubble. On the basis of comparison with previous work, picosecond laser pulses are thought to be the most efficient way to cause laser-induced size reduction of gold nanoparticles.

1. Introduction The interaction between light and nanoparticles is of great interest because nanoparticles often show unique optical properties originating in their high ratio of surface area to volume and the quantum size effects. Nonlinear optical devices1 and holographic gratings2 are good examples of their applications. Gold nanoparticles show distinct absorption in the visible region due to surface plasmon resonance (SPR), and their fundamental optical properties are studied using continuous white light.3-7 On another front, pulsed laser light enables us to inject much energy into materials and expand the research field. Studies on ultrafast energy relaxation dynamics in gold nanoparticles induced by femtosecond laser pulses are good examples.8-14 Irradiation of intense pulsed laser light to gold nanoparticles induces their size reduction,15-32 size enlargement,19,33 and morphological changes.33-38 The photothermal process in which the absorbed photon energy heats the particles in a few picoseconds8-12 is considered to cause enlargement and morphological changes.33,36,37 Morphological changes of ellipsoidal gold nanoparticles with low aspect ratios to spheres occurred ca. 100 °C lower than the melting point because of surface melting.38 However, for laser-induced size reduction, two kinetic processes are proposed; one is the vaporization of particles induced by a photothermal process in which particles vaporize at the boiling point,15 and the other is a kind of Coulomb explosion of particles induced by electron ejection from particles.18,30 For femtosecond laser irradiation to gold nanorods, experimental results by Link and co-workers are in contrast with the hypothesis based on the photothermal process since the size * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemical System Engineering. ‡ Department of Electronic Engineering.

reduction of gold nanorods occurred without correlating with the fact that the temperature of the particles reached the boiling point.30 However, Inasawa et al. suggested a semiquantitative model that explained the maximum diameter of particles, existing in the system after sufficient laser irradiation, reported by Takami et al..15,21 Their model was based on the heat balance, including the heat dissipation from a particle to the surrounding media, because the heat dissipation occurred in 100-200 ps11 (shorter than the pulse width used by Takami et al.; they used nanosecond laser pulses15). Their work supports the photothermal process as its mechanism. In addition to the contrasting experimental results above, phenomenological details, such as to what extent particles reduce their sizes per one pulse and what is the main factor determining the size of small particles, which are generated by laser-induced size reduction, are not yet known, because previous work has been interested only in the comparison between before and after enough laser irradiation onto the particles. Thus, most of the questions about the kinetic process of laser-induced size reduction of gold nanoparticles still remain to be answered. In this report, to find out the kinetic process of the laserinduced size reduction, we carefully observed the evolution of the size distribution of gold nanoparticles after irradiation of several pulses by transmission electron microscopy (TEM). We used picosecond laser pulses to minimize the energy dissipation out of the particles. After irradiation of several pulses was applied to monomodal gold nanoparticles, the size distribution turned into a bimodal one. Similar size distributions were shortly reported by Link and co-workers, but the details are unknown.30 There are two peaks in the number of particles, at 6 and 1624 nm. Large particles, which were the results of the sizereduced initial particles, gradually reduced their sizes, while only particles smaller than 12 nm were formed by the laser-induced

10.1021/jp0441240 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/14/2005

Bimodal Size Distribution of Gold Nanoparticles

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size reduction. Focusing on the evolution of the mean diameter of particles larger than 12 nm in terms of the total number of shots, we extracted a characteristic value for the size-reduction rate per one pulse and proposed a mechanism based on the photothermal process. Finally, we compare our results with previous work and summarize the laser-induced size-reduction phenomenon. 2. Experimental Section Gold nanoparticles were prepared by chemical reduction in aqueous solution.39 A 50 mL aliquot of 0.12 mM AuCl4- ion aqueous solution prepared by dissolving HAuCl4‚4H2O (Wako Chemical, 99.9%) yellow solid in pure water was heated, and 180 µL of 0.1 M citric acid (Wako Chemical) aqueous solution was added at the boiling stage with vigorous stirring. The solution soon turned wine-red, indicating the production of gold nanoparticles. Through the use of this method, all AuCl4- ions were reduced to gold nanoparticles. The mean diameter of the prepared gold nanoparticles was 25 nm in ellipsoidal shapes with aspect ratio of ca. 1.3. This aqueous solution was stable for more than 1 month in the refrigerator. The solution was further diluted six times with purified water just before the laser irradiation experiments. For simplicity of the particle size analysis, we changed the particles’ shapes into spherical by the laser treatment; chemically prepared particles were irradiated with weak laser energy (5 mJ cm-2 pulse-1) for 10 min at 10 Hz to make all the particles spherical. This operation was performed just before the laser-induced size-reduction experiments. A third harmonic Nd:YAG laser (EKSPLA, PL2143B) (λ ) 355 nm, pulse width 30 ps) was used for laser irradiation of the aqueous solution of gold nanoparticles. A 50 µL aliquot of the aqueous solution of gold nanoparticles was introduced into a quartz cell (2 × 10 × 45 mm3, optical path 10 mm). The top of the liquid level was 2 mm from the bottom. The spot size of the irradiated laser was 9 mm in diameter, which was larger than the cross section of the aqueous solution (2 × 2 mm2), and the aqueous solution was irradiated by the center of the laser spot. Absorption spectra were recorded before and after the pulsed laser irradiation, using a multichannel detector (Hamamatsu, PMA-11), D2, and a W lamp (Hamamatsu, L7893). Irradiated laser energy was measured with a power meter (Scientech, model 362). For direct observation of the gold nanoparticles, we used TEM (JEOL, JEM-2010) operating at 200 kV. TEM images were recorded by a CCD camera (Gatan, model 794). High-resolution TEM (HRTEM) images were also taken (JEOL, JEM-4000FXII) operating at 400 kV. A drop of the aqueous solution of gold nanoparticles was put onto a carbon-coated copper TEM grid, letting it dry completely. More than 500 gold nanoparticles were observed to obtain distributions of their sizes. 3. Results and Discussion 3.1. Bimodal Size Distribution of Gold Nanoparticles. Figure 1 shows spectra of the aqueous solution recorded after laser irradiation. The solution after the laser treatment at 5 mJ cm-2 pulse-1 shows distinct absorption due to the SPR of the gold nanoparticles (Figure 1a). Further irradiation of laser pulses with laser energy of 23 mJ cm-2 pulse-1 after the laser treatment causes a decrease in absorption intensity as seen in Figures 1bd. With the increase in the total number of shots, the absorption becomes weaker and broader, and the peak position shifts to a shorter wavelength. Since the absorption due to the SPR of the

Figure 1. Absorption spectra of the solution, after the laser treatment (a) and after further irradiation with laser pulses (b-d). Irradiation conditions were 5 mJ cm-2 pulse-1 at 10 Hz for 10 min (a) and 4, 10, and 50 pulses with a laser energy of 23 mJ cm-2 pulse-1 for parts b, c, and d, respectively.

Figure 2. TEM images of gold nanoparticles, before (a), after the laser treatment (b), and after further irradiation of laser pulses (c-e). Irradiation conditions were 5 mJ cm-2 pulse-1 for 10 min at 10 Hz (b), 4, 10, and 50 pulses with a laser energy of 23 mJ cm-2 pulse-1 for parts c, d, and e, respectively. Scale bars in all images are 20 nm.

gold nanoparticles becomes weaker and broader with a decrease of their sizes,5,23-28 it is considered that these absorbance changes observed in Figures 1b-d are caused by the size reduction of the gold nanoparticles. The blue shift of the peak position is also consistent with previous work by Takami and co-workers.15 TEM images of gold nanoparticles before and after the pulsed laser irradiation are shown in Figure 2. Chemically prepared gold nanoparticles are ellipsoidal in shape with some sharp edges as seen in Figure 2a. After the laser treatment at 5 mJ cm-2 pulse-1, most nanoparticles become spherical (Figure 2b). No small particles produced by laser-induced size reduction were observed in Figure 2b, and we concluded that the laser treatment at 5 mJ cm-2 pulse-1 did not induce the size reduction of gold nanoparticles. When the images shown in Figures 2b-e were compared, the laser-induced size reduction of gold nanoparticles with a laser energy of 23 mJ cm-2 pulse-1 is clearly confirmed. Additional irradiation with laser pulses increases the number of small particles and reduces the size of large particles (Figures 2c-e). Interestingly, the diameters of most small particles are smaller than ca. 10 nm, while those of large particles are larger than 15 nm (Figures 2c-e). From the images in Figures 2c-e, the size distribution of the gold nanoparticles seems to become bimodal under the pulsed laser irradiation.

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Figure 3. Size distributions of gold nanoparticles before (a) and after irradiation of 4, 10, and 50 pulses for parts b, c, and d, respectively. The laser energy was 23 mJ cm-2 pulse-1 for all cases. Note that only spherical particles larger than 4 nm were counted.

Figure 3 shows the size distributions of gold nanoparticles before and after the irradiation with several pulses with a laser energy of 23 mJ cm-2 pulse-1. In Figure 3, particles smaller than 4 nm were not counted. Even if 1000 particles with diameters of 4 nm exist, the net volume fraction of such small particles is 5% at most. Therefore, neglecting particles smaller than 4 nm scarcely affects the mass balance of the particles. Furthermore, because our objective is to show the modal change of size distribution into a bimodal one and to analyze the sizereduction rate of large particles per one pulse, the mass balance is not necessarily required. The initial monomodal size distribution (Figure 3a) turned into a bimodal one after irradiation with several pulses (Figures 3b-d). These bimodal size distributions of gold nanoparticles under nanosecond laser pulses were shortly reported by Link and co-workers.30 Two peaks exist in the number of particles, one at 6 nm and the other at 16-24 nm with a “valley” around 12 nm. The distribution, which consists of large particles, is the result of size-reduced initial particles, whereas the volume removed from large particles is converted to smaller particles. After the irradiation with 50 pulses, the peak position of the distribution for large particles is at 16 nm (Figure 3d). With further irradiation of pulsed laser light, we hardly observed particles larger than 12 nm in diameter. Almost all of the initial particles are thought to reduce their size and become particles smaller than 12 nm in diameter with a sufficient number of pulses, resulting in the monomodal size distribution with a smaller diameter once again. Therefore, the bimodal size distribution observed here is interpreted as a transient phenomenon that can be seen before the completion of the size reduction of initial particles into particles smaller than 12 nm under laser pulses. In Figure 3, although the peak position of the distribution for large particles shifts to a smaller diameter with the number of pulses, the position of the valley hardly changes. This fact indicates that only particles smaller than 12 nm can be produced as a result of the laser-induced size reduction of large particles. Figure 4 shows the size distributions of gold nanoparticles after the irradiation with several pulses with different laser energies. The peak position of the distribution for large particles rapidly shifts to smaller diameters when irradiated with intense pulses. Regarding the position of the valley, it hardly varies

Inasawa et al.

Figure 4. Size distributions of gold nanoparticles after irradiation with several pulses. Irradiation conditions are 10 pulses with 14 mJ cm-2 pulse-1 (a), 20 pulses with 14 mJ cm-2 pulse-1 (b), 4 pulses with 43 mJ cm-2 pulse-1 (c), and 10 pulses with 43 mJ cm-2 pulse-1 (d). Only spherical particles larger than 4 nm were counted.

despite the difference of laser energies. The valley position is at around 12 nm for all cases. That is to say that the diameters of small particles produced by laser-induced size reduction are independent of the irradiated laser energy. In other words, the diameters of small particles produced by size reduction do not depend on the reduction rate of large particles. A similar trend was observed in previous work using nanosecond laser pulses.30 Two possible explanations are considered for this size-limited formation of small particles and the shift of the peak position of the size distribution of large particles; one is a gradual decrease of large particles in their sizes due to the partial vaporization of particles followed by the cooling and condensation of gaseous gold into small particles (the photothermal process), and the other is a kind of Coulomb explosion of large particles into many small particles due to the ionization of particles. Considering previous findings that (1) laser-induced size reduction of gold nanorods, which is not correlated with a photothermal process, occurs with a laser energy 3 orders higher than ours,30 (2) while Coulomb explosion of silver nanoparticles was reported under picosecond laser pulses with nearly the same laser energy as ours18 and the work function of gold (5.3 eV)40 is larger than the value for silver (4.6 eV),40 we can neglect the possibility that gold nanoparticles reduce their sizes by Coulomb explosion due to the ionization of particles. The kinetics of laserinduced size reduction of gold nanoparticles are discussed below. 3.2. Kinetic Study of the Laser-Induced Size Reduction of Gold Nanoparticles. In this subsection, we discuss the kinetics of the laser-induced size reduction of gold nanoparticles. Figure 5 shows the HRTEM images of gold nanoparticles after single-pulse laser irradiation with a laser energy of 43 mJ cm-2 pulse-1. Interestingly, protruding parts such as twisted strings are formed on large particle surfaces, and the diameters of such large particles are around 22-23 nm, which is smaller than the mean diameter of 25 nm (Figures 5a and 5b). Although it can be considered that the particles in Figure 5 are the results of the attachment of small particles to larger ones during the drying process for TEM, in this case, small particles should randomly attach to larger ones, and the formation of gold strings on large particles is less expected. In addition, no clear grain boundary is seen between the large particle surfaces and the gold strings

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Figure 5. HRTEM images of size-reduced gold nanoparticles after single-pulse irradiation with a laser energy of 43 mJ cm-2 pulse-1. Parts c and d are high-magnification images of parts of a and b, indicated by arrows, respectively. No clear grain boundary is observed between the particles’ surfaces and the gold strings in parts c and d. Scale bars are 10 nm. Details are in the text.

(Figures 5c and 5d). Therefore, it is considered that the particles in Figure 5 are not produced by the attachment of small particles onto the surface of larger ones during the drying process. We consider that these particles are formed by the projection of some volume from the inside of the large particles due to a kind of boiling, and consequently the diameters of these particles are relatively small. Coulomb explosion of gold nanoparticles cannot explain the formation of such particles, because particles should only burst out into small particles in that case. The images in Figure 5 also support the photothermal process as the mechanism for the laser-induced size reduction of gold nanoparticles in our system. Therefore, we consider the mechanism on the basis of the photothermal process below. Paying attention to the size-reduction rate of large particles, we proceed to a quantitative discussion based on the evolution of the mean diameter of particles larger than 12 nm. Figure 6 shows the evolution of the mean diameter for large particles in terms of the total number of shots. Particles larger than 12 nm in diameter are counted in Figure 6. Error bars in Figure 6 mean the standard deviation of the mean diameter not the standard deviation of the size distribution of particles larger than 12 nm. Irradiation with high energy makes the size-reduction rate larger. From Figure 6, we can extract a characteristic value describing the size-reduction rate. Assuming that large particles reduce their sizes with a constant volume fraction η per one pulse, then the radius of large particles after the irradiation with n pulses is easily written as

Rn ) (1 - η)n/3R0

(1)

where Rn is the radius of a particle after irradiation of n pulses and R0 is the initial radius of the particle. In Figure 6, fitting results based on eq 1 with η as a single fitting parameter are also shown and in good agreement with experimental data. Obtained values of η are 5.7 × 10-3 for a 14 mJ cm-2 pulse-1, 2.3 × 10-2 for a 23 mJ cm-2 pulse-1, and 1.1 × 10-1 for a 43 mJ cm-2 pulse-1, respectively.

Figure 6. The evolution of the mean diameter for large particles in terms of the total number of shots with various laser energies: 14 mJ cm-2 pulse-1 (circles), 23 mJ cm-2 pulse-1 (squares), and 43 mJ cm-2 pulse-1 (diamonds). Particles larger than 12 nm in diameter were considered to obtain the mean diameter. Solid lines are fitting results based on eq 1 with a single fitting parameter of η. Note that error bars mean the standard deviation of the mean diameter not the standard deviation of the size distribution of particles larger than 12 nm.

Here, we estimate the photon energy absorbed by particles. Since by the measurement of laser energy before and after the sample solution, we confirmed that the absorption of pulsed laser light by the sample solution was in accordance with the Lambert-Beer law and only 7.7% of laser light was absorbed by the aqueous solution; absorbed photon energy, Eabs, is simply expressed as

Eabs )

I(1 - 10-A) CAul0

(2)

where I is laser energy, A is the absorbance at 355 nm, CAu is the atomic concentration of the aqueous solution of gold nanoparticles, and l0 is the optical path. From eq 2, Eabs is estimated to be 5.4 × 104 J mol-1 for the I ) 14 mJ cm-2 pulse-1 under the conditions that A ) 0.035 (Figure 1), CAu ) 2 × 10-8 mol cm-3, and l0 ) 1 cm. The values of Eabs for I ) 23 and 43 mJ cm-2 pulse-1 were obtained by the same procedure. The obtained values of η are plotted in terms of the absorbed photon energy in Figure 7. Experimental data for η without irradiation and with the laser treatment (I ) 5 mJ cm-2 pulse-1) are also plotted. For comparison, the energy required for the heating of particles up to the boiling point, Eb.p.(8.4 × 104 J mol-1), and the energy required for the complete vaporization, Evap(4.0 × 105 J mol-1), are estimated under the assumption that the heat capacity, Cp(25.4 J mol-1 K-1), the enthalpy of fusion, ∆Hfusion(1.3 × 104 J mol-1), and the enthalpy of vaporization, ∆Hvap(3.2 × 105 J mol-1), are the same as the bulk values and the boiling point is 3130 K.40 Note that Evap is the sum of two values, Eb.p. and ∆Hvap. As already described, the possibility of ionization of gold nanoparticles is negligible. In addition, because the time constant for the melting of gold nanorods is on the order of 30 ps35 and vaporization is considered to occur after the melting, the time constant for the size-reduction phenomenon due to vaporization is considered to be larger than 30 ps. Therefore, we consider that sizereduction phenomenon does not occur during the pulse duration of our laser. Furthermore, since the time constant for the heat dissipation from particles (100-200 ps11) is also larger than the pulse width of our laser, we can directly compare the

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Inasawa et al.

Figure 7. Dependence of η as a function of absorbed photon energy, Eabs. Required energy for heating of gold up to the boiling point, Eb.p., and complete vaporization, Evap, are also shown.

absorbed photon energy calculated in eq 2 with Eb.p. and Evap. In Figure 7, while Eabs is larger than Eb.p. for I ) 23 and 43 mJ cm-2 pulse-1, Eabs is smaller than Eb.p. for I ) 14 mJ cm-2 pulse-1, meaning that the temperature of the particles does not reach the boiling point by the irradiation of laser pulses under such conditions. In Figure 8, the values of η are plotted in terms of the temperatures of the particles. We converted Eabs into the temperature of the particles just after a pulse irradiation by simple equations described below,15 if

Eabs < Em.p. T)

Eabs + TR Cp

(3)

if

Em.p. < Eabs < Em.p. + ∆Hfusion T ) Tm.p.

(4)

if

Em.p. + ∆Hfusion < Eabs < Eb.p. T)

Eabs - ∆Hfusion + TR Cp

(5)

and if

Eb.p. < Eabs < Evap T ) Tb.p.

(6)

where Em.p.(2.6 × 104 J mol-1) is the energy required for the heating of gold up to the melting point, Tm.p.(1064 °C) and Tb.p. are the melting point and the boiling point of gold, respectively, and TR is the room temperature (25 °C). The temperature of the gold nanoparticles is about 1740 °C for a I ) 14 mJ cm-2 pulse-1 and is below the boiling point. Results in Figures 7 and 8 show that boiling is not necessarily required for the sizereduction phenomenon. Size reduction below the boiling point can be explained by the gold vapor layer formed around liquid gold drops. According to classical thermodynamics, the vapor pressure at the liquid interface in equilibrium is expressed by

Figure 8. η versus calculated temperature of gold nanoparticles. Tm.p. and Tb.p. are the melting point and the boiling point of gold, respectively.

the Clausius-Clapeyron equation41 as

(

P ) P0 exp -

(

∆Hvap 1 1 R T Tb.p.

))

(7)

where P and P0 are the vapor pressure at temperatures T and Tb.p., respectively, and R is the gas constant. Through the use of eq 7, the vapor pressure P at 1740 °C is estimated as 1.1 × 102 Pa, using P0 ) 1.0 × 105 Pa, ∆Hvap ) 3.2 × 105 J mol-1, and Tb.p. ) 3130 K.40 The curvature effect of gold nanoparticles gives about a 10% larger pressure than the value obtained above when the Kelvin equation is considered.41 That is to say that after the laser heating it is considered that gold vapor is formed around hot liquid gold drops even below the boiling point. Such formation of gold vapor is thought to result in the size reduction of gold nanoparticles below the boiling point. In other words, the important factor is not the temperature itself but the vapor pressure at the liquid interface. Our understanding of the laser-induced size reduction of gold nanoparticles is shown in Figure 9. Even when absorbed photon energy is insufficient for the boiling, a vapor layer of gold is formed due to the vapor pressure of liquid gold (Figure 9a). Because a water vapor layer is formed around hot liquid gold drops,42 surrounding water is not considered to disturb the formation of gold vapor. The amount of gold vapor depends on the vapor pressure at the interface. After being cooled, some vaporized gold atoms go back to the “mother” particles while some are condensed into small particles. With sufficient laser energy for the boiling, gold vapor can be also formed inside liquid gold drops because both liquid and vapor can coexist at the boiling stage (Figure 9b). Gold bubbles in liquid gold go to the outside of liquid drops, resulting in the projection of some volume of liquid gold around the bubble (Figure 9b). Therefore, after being cooled, particles with gold strings in Figure 5 are formed. Such gold strings also turn into small particles by additional pulses. Considering the fact that the diameters of the small particles are almost independent of the irradiated laser energy (Figures 3 and 4), the condensation kinetics of gold vapor are thought to be one of the main factors for the formation of small particles with size restriction. Small particles generated by size reduction are less expected to reduce their sizes because the time constant of heat dissipation is proportional to the square of the radius (10 ps for 4 nm and 400 ps for 50 nm diameters).43 Hence, though small particles also absorb photon energy, they are not heated to high temperature due to the heat dissipation.

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Figure 9. Schemes for the laser-induced size reduction of gold nanoparticles by a photothermal process.

3.3. Comparison with Previous Work. Two typical works were performed on the laser-induced size reduction of gold nanoparticles; one was by Takami et al.,15 and the other by Link et al.30 Takami and co-workers discovered laser-induced size reduction of gold nanoparticles under nanosecond laser pulses (pulse width 7 ns) and explained the mechanism as a photothermal process.15 In their report, the maximum diameter of the particles, which exist in the system after enough irradiation, is explained in terms of the temperature of the particles.15 They estimated that the temperature of a particle increased above several thousand Kelvin after single-shot irradiation under the assumption that no heat dissipation from the particle occurred.15 However, some counterarguments were made against their interpretation. The point of such counterarguments is that the time constant for the heat dissipation from a particle to solvent is 100-200 ps;11 therefore their assumption is invalid in the case of the irradiation of nanosecond laser pulses.30,44 In their calculation, the lattice temperature for a 5 ns pulse was about 20 times lower in degrees Celsius than the one for a 30 ps pulse with the same total energy.44 However, recent work has revealed that when water contacts with a hot gold surface (1000 K), a water vapor layer is formed in the vicinity of the surface in about 100 ps,42 and such a formation of a vapor layer is considered to make the heat dissipation from particles much slower due to the lower thermal conductivity of vapor than of water.40,45,46 Our previous work made a semiquantitative model to explain the experimental results by Takami et al.15 based on the heat balance of a particle including the heat dissipation from a particle with the thermal conductivity of vapor,21 which is consistent with the formation of a water vapor layer. Furthermore, it was reported that the extremely high lattice temperature of gold nanoparticles (∼1000 K) is maintained for well over 100 ps in water.45 Therefore, it is considered that the heat dissipation from particles is not negligible, but the high temperature of a particle can be maintained if nanosecond laser pulses with high energy are irradiated because a water vapor layer is formed around the particles and disturbs the heat

dissipation. From these considerations, even though the estimation by Takami and co-workers is somewhat coarse, the mechanism of size reduction with nanosecond laser pulses is thought to be a photothermal process after all. Link et al. reported laser-induced size reduction of gold nanorods caused by multiphoton ionization.30 They used a laser with a pulse width of 100 fs, which was nearly on the same order as the time constant for electron-electron scattering47 but much shorter than the time constant for electron-phonon scattering.8-12,30,48 Therefore, in their system, absorbed photon energy is transferred only to the electrons, and electron ejection can occur. This electron ejection is considered to result in insufficient heating of the nanorods.30 Hence, through the use of femtosecond laser pulses, the mechanism for laser-induced size reduction of gold nanoparticles is not a photothermal process but electron ejection followed by the Coulomb explosion.30 The important point that determines the mechanism is whether electrons can absorb enough energy to be ejected or not. In the case of the irradiation of laser pulses with pulse widths shorter than the time constant for electron-phonon scattering, only electrons are excited by absorbed photons to extremely high-energy states, resulting in electron ejection. In the case of a longer pulse width than the time constant for relaxation to the lattice, absorbed photon energy transfers to the phonon during the irradiation of a pulse, and electrons are not at high-energy states. In this case, electron ejection is less expected, and the photothermal process becomes dominant. The laser-induced phenomenon in gold nanoparticles depends on the pulse width of the laser light as reported by Link et al.30 From the viewpoint of irradiated laser energy, the energy required for size reduction with femtosecond laser pulses is higher by about 3 orders magnitude than that in our study.30 This means that for the Coulomb explosion of gold nanoparticles the ejection of an enormous number of electrons seems to be required. That is to say that to induce the size reduction of gold nanoparticles by multiphoton ionization using femtosecond laser pulses is not efficient. Nearly the same laser energy is required for the size

9410 J. Phys. Chem. B, Vol. 109, No. 19, 2005 reduction with nanosecond laser pulses, because the absorbed laser energy is lost by the heat dissipation from the particles. For the laser-induced size reduction of gold nanoparticles, because the size reduction is caused by a photothermal process and the heat dissipation from particles is negligible, the irradiation of picosecond laser pulses is supposed to be the most efficient method. 4. Conclusions Through the use of picosecond laser pulses, the kinetics for the size reduction of gold nanoparticles were studied. The initial monomodal size distribution of gold nanoparticles changed into a bimodal one during pulsed laser irradiation. This modal change was caused by the gradual size reduction of gold nanoparticles. From careful observation by TEM, we extracted a characteristic value for the size-reduction rate per one pulse. It was revealed that the size reduction of gold nanoparticles under picosecond laser pulses occurred even below the boiling point because of the gold vapor formation around hot liquid gold drops. With sufficient laser energy for the boiling of particles, gold vapor can also be formed inside liquid gold drops, and such bubbles go to the outside of the drops with some volume of liquid gold around the bubbles, resulting in the formation of particles with gold strings on their surfaces. Our results also indicate that the important factor that determines the phenomenon is not temperature but the vapor pressure at the liquid interface. The condensation process of gold vapor is thought to be one of the main factors that restricts the sizes of small particles. Finally, we summarized the laser-induced size reduction of gold nanoparticles and suggested that picosecond laser pulses were the most efficient way to induce the size reduction of gold nanoparticles. Acknowledgment. This work was supported by the “Nanotechnology Materials ProgramssNanotechnology Particle Project” of the New Energy and Industrial Technology Development Organization based on funds provided by the Ministry of Economy, Trade and Industry, Japan. We also thank the High Voltage Electron Microscope Laboratory, School of Engineering, The University of Tokyo, for their technical support for TEM. References and Notes (1) Hamakawa, Y.; Fukuta, K.; Nakamura, A.; Liz-Marzan, L. M.; Mulvaney, P. Appl. Phys. Lett. 2004, 84, 4938. (2) Hirose, T.; Omatsu, T.; Sugiyama, M.; Inasawa, S.; Koda, S. Chem. Phys. Lett. 2004, 390, 166. (3) Mie, G. Ann. Phys. 1908, 25, 377. (4) Doremus, R. H. J. Chem. Phys. 1964, 40, 2389. (5) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (6) Palpand, B.; Prevel, B.; Lerme, J.; Cottancin, E.; Pellarin, M.; Treilleux, M.; Perez, A.; Vialle, J. L.; Broyer, M. Phys. ReV. B 1998, 57, 1963. (7) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (8) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (9) Link, S.; Burda, C.; Mohamad, M. B.; Nikoobakht, B.; El-Sayed, M. A. Phys. ReV. B 2000, 61, 6086. (10) Logunov, S. L.; Ahmadi, T. S.; El-Sayed, M. A.; Khoury, J. T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3713.

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